Java lambda表达式详解

Java lambda表达式是一种很有用很方便很常见的语法，先据一个例子：

public class Java8Tester {
   public static void main(String args[]){
      Java8Tester tester = new Java8Tester();
		
      //with type declaration
      MathOperation addition = (int a, int b) -> a + b;
		
      //with out type declaration
      MathOperation subtraction = (a, b) -> a - b;
		
      //with return statement along with curly braces
      MathOperation multiplication = (int a, int b) -> { return a * b; };
		
      //without return statement and without curly braces
      MathOperation division = (int a, int b) -> a / b;
		
      System.out.println("10 + 5 = " + tester.operate(10, 5, addition));
      System.out.println("10 - 5 = " + tester.operate(10, 5, subtraction));
      System.out.println("10 x 5 = " + tester.operate(10, 5, multiplication));
      System.out.println("10 / 5 = " + tester.operate(10, 5, division));
		
      //without parenthesis
      GreetingService greetService1 = message ->
      System.out.println("Hello " + message);
		
      //with parenthesis
      GreetingService greetService2 = (message) ->
      System.out.println("Hello " + message);
		
      greetService1.sayMessage("Mahesh");
      greetService2.sayMessage("Suresh");
   }
	
   interface MathOperation {
      int operation(int a, int b);
   }
	
   interface GreetingService {
      void sayMessage(String message);
   }
	
   private int operate(int a, int b, MathOperation mathOperation){
      return mathOperation.operation(a, b);
   }
}

　　可以看出Java lambda表达式的形式为 parameter -> expression。上面程序中如果不用lambda表达式，也可以实现，比如用类实现接口，然后生成对象，再调用之类的，或者好一点，也是用一个匿名类，但是这样会非常麻烦，远远没有lambda表达式简洁.

再看另一个例子。如果有一个Person列表，Person包含年龄，性别，名字等字段，现在要写一个根据条件过滤这个列表并打印出来等程序，用lambda表达式可以这么做：

 

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Predicate;

class Person {

    public enum Sex {
        MALE, FEMALE
    }

    String name;

    Sex gender;

    int age;

    String emailAddress;

    public int getAge() {
        return age;
    }

    public Sex getGender() {
        return gender;
    }

    public void setGender(Sex gender) {
        this.gender = gender;
    }

    public void printPerson() {
        System.out.println( "The name is: "+ name + " at age: "+ age);
    }

    public Person(String name, Sex gender, int age, String emailAddress) {
        this.name = name;
        this.gender = gender;
        this.age = age;
        this.emailAddress = emailAddress;
    }
}

public class Main {
    public static void printPersonsWithPredicate(List<Person> roster, Predicate<Person> tester) {
        for (Person p : roster) {
            if (tester.test(p)) {
                p.printPerson();
            }
        }
    }

    public static void processPersons(
            List<Person> roster,
            Predicate<Person> tester,
            Consumer<Person> block) {
        for (Person p : roster) {
            if (tester.test(p)) {
                block.accept(p);
            }
        }
    }

    public static void main(String[] args){
        List<Person> pl = new ArrayList();
        pl.add(new Person("Jet", Person.Sex.MALE, 21, "Jet@163.com"));
        pl.add(new Person("Mike", Person.Sex.MALE, 18, "Mike@163.com"));
        pl.add(new Person("Jim", Person.Sex.MALE, 28, "Jim@163.com"));
        pl.add(new Person("Lucy", Person.Sex.FEMALE, 20, "Lucy@163.com"));
        pl.add(new Person("Diana", Person.Sex.FEMALE, 28, "Diana@163.com"));

        printPersonsWithPredicate(
                pl,
                p -> p.getGender() == Person.Sex.FEMALE
                        && p.getAge() >= 18
                        && p.getAge() <= 25
        );

        System.out.println("==========================");

        processPersons(
                pl,
                p -> p.getGender() == Person.Sex.MALE
                        && p.getAge() >= 18
                        && p.getAge() <= 25,
                p -> p.printPerson()
        );
    }
}

 

 

可以看到这里分别调用了Java的标准接口Predicate和Consumer，返回类型不同需要注意,然后我们用lambda表达式逻辑上实现它的test方法和accept，就可以正确使用了，是不是很方便？